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Dissociating size representation for action and for conscious judgment: Grasping visual illusions without apparent obstacles
Consciousness and Cognition Consciousness and Cognition 15 (2006) 269–284 www.elsevier.com/locate/concog
Dissociating size representation for action and for conscious judgment: Grasping visual illusions without apparent obstacles Elisabeth Sto¨ttinger, Josef Perner * Department of Psychology, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria Received 12 May 2005 Available online 9 September 2005
Abstract Visual illusions provide important evidence for the co-existence of unconscious and conscious representations. Objects surrounded by other figures (e.g., a disc surrounded by smaller or larger rings, Ebbinghaus/ Titchener illusion) are consciously perceived as different in size, while the visuo-motor system supposedly uses an unconscious representation of the discsÕ true size for grip size scaling. Recent evidence suggests other factors than represented size, e.g., surrounding rings conceived as obstacles, affect grip size. Use of the diagonal illusion avoids visual obstacles in the path of the reaching hand. Results support the dual representation theory. Grip size scaling follows actual size independent of illusory effects, which clearly bias conscious perception in direct comparisons of lengths (Experiment 1) and in finger-thumb span indications of perceived length (Experiment 2). 2005 Elsevier Inc. All rights reserved. Keywords: Visual illusions; Perceptual judgement; Knowledge for action; Implicit knowledge; Conscious perception; Obstacles avoidance
1. Introduction The theory that it is possible to have visually gained knowledge without conscious awareness has been greatly helped by the finding from blindsight patients (e.g., Perenin & Rossetti, 1996; *
Corresponding author. E-mail address: [email protected] (J. Perner).
1053-8100/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.concog.2005.07.004
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Po¨ppel, Held, & Frost, 1973; Weiskrantz, Sanders, & Marshall, 1974; Weiskrantz, 1998). Despite complete lack of conscious perception of stimuli in the ÔblindÕ part of their visual field they can, nevertheless, fairly accurately point to the stimulus or accurately act in other ways in relation to it. Support for the existence of similar dissociations in neurologically healthy people comes from experiments with illusory stimuli. Early demonstrations relied on motion illusions using the induced Roelofs Effect investigating eye gaze (Wong & Mack, 1981) and manual action (Bridgeman, Gemmer, Forsman, & Huemer, 2000; Bridgeman, Kirch, & Sperling, 1981). More recently, Aglioti, DeSouza, and Goodale (1995) used the Ebbinghaus/Titchener circles illusion to show a corresponding dissociation. When asked to indicate which of two discs of the same size was larger, the one surrounded by smaller rings was judged larger than the one surrounded by larger rings— in keeping with the known effect of this illusion. However, the grip size scaling (thumb finger opening on the way to grasping a disc) was about the same, indicating that the sensory-motor system allegedly uses a different—more accurate—representation (knowledge) of the discsÕ true sizes than conscious perception. This interpretation of the original finding by Aglioti et al. (1995) has, however, come in for much criticism. We concentrate here on the question whether the results of this experiment or any of its later variations requires for their explanation the existence of two contradictory representations of reality (dual representations model)—one of which is available to conscious awareness and the other one is not—or whether they can be explained on the basis of a single, conscious representation (single representation model). For answering this question the following aspects of the experimental setup and model of relevant mental processes are critical. In the basic setup a particular stimulus object is presented and one of its properties (e.g., its size) is assessed by two different response modes (e.g., a verbal judgment and a grasping action) which yield contradictory response mode information about the stimulus property. The dual representations model assumes that the contradictory information in the different response modes comes from different representations. However, the single representation model can also explain this contradictory response mode information if one of the following two possibilities arises: 1. The mapping from stimulus to internal representation is not constant over the two tasks in which each response mode is used. Pavani, Boscagli, Benvenuti, Rabuffetti, and Farne` (1999) and, in particular, Franz, Gegenfurtner, Bu¨lthoff, and Fahle (2000) have capitalized on this possibility by observing that the size of the illusory effect is much stronger when the two discs in each display, one disc surrounded by smaller the other disc by larger rings, are directly compared than when a disc in one of these displays is compared to a plain disc (without any surrounding rings). Participants in AgliottiÕs experiment had to judge size by comparing both displays, resulting in relatively large differences between represented and actual size. In contrast, participants had to grasp only one of the two discs, which might have restricted their attention to only that particular disc, attenuating the illusory effect and, consequently, resulting in seemingly more accurate grip size differences. Hence, on each occasion only a single representation of each discÕs size might be formed, but due to the attentional differences (attend to both displays for judgment but only to one when grasping) that representation is more accurate in the grasping than in the judgment task. Consequently, the single representation theory can adequately explain the difference in manually and verbally expressed size. 2. Extraneous reasons for acting may bias the action response. When asked to give a conscious judgment of the size of a disc then the best estimate available is the size as internally represented.
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In contrast, when grasping the disc there may be different valid reasons for a particular fingerthumb span (Jacob & Jeannerod, 2003; Smeets & Brenner, 1999). The actual size of the disc will only be one of them. Another good reason is the space between the disc and surrounding obstacles. If the disc is presented on its own, one can afford a fairly large maximum grip span that leisurely narrows in on the true size of the disc as the hand approaches. If, however, there is barely space to fit oneÕs fingers between the disc and surrounding objects then one will adjust oneÕs grip size to a minimum above the size of the disc. In fact, Haffenden and Goodale (2000) and Haffenden, Schiff, and Goodale (2001) found that the visuo-motor system seems to make exactly such adjustments—even when the surrounding obstacles are only two-dimensional marks on the supporting surface (and strictly speaking are not real obstacles). Haffenden and Goodale (2000) used an additional ‘‘new’’ Ebbinghaus figure where the gap between target and surrounding small circles was the same as between target and surrounding large circles. They found that the critical variable for grasping was not the perceived size of the target but the distance between target and context circles. However, Franz, Bu¨lthoff, and Fahle (2003) did a similar experiment with another additional Ebbinghaus figure (the same gap between target and surrounding large circles as between target and surrounding small circles). They found a systematically increasing grip aperture with perceived size—independent of the gap between target and context circles. In their experiment participants had to grasp the target circle of the four context conditions with their dominant hand. As soon as participants started their grasping movements, shutter glasses suppressed their vision preventing any kind of visual feedback throughout the entire movement. Participants had four seconds to carry out this movement, which creates a problem of interpretation pointed out by Millner and Goodale (1995): the unconscious representation for guiding movements does not persist for longer than 2 s. If the delay between seeing an object and movement execution toward this object is longer than 2 s, movements are based on the longer lasting conscious perceptual representation, as found by Goodale, Jakobson, and Keillor (1994) with the visual form agnosia patient D.F. See also Bridgeman, Peery, and Anand (1997), Marcel (1993). Hence, if participants in the experiment by Franz et al. (2003) did take close to 2 s for their movement with shutter glasses, it is fairly likely that these authors found an influence of the illusion on grasping because the motor representation had decayed and participants were forced to base their grasping on the illusory perceptual representation. In any case, the Ebbinghaus Illusion is an inadequate method for deciding whether grasp apertures depends on the size of the objects to be grasped, because the object is surrounded by context circles (obstacles) and the gap between target and small and large context circles can never be matched completely. It remains an irresolvable question as to how to control the distance between target disc and context circles: is it to be controlled at the narrowest point or in terms of the area between two context circles and the target object, etc. Other illusory stimuli that have been used in this area suffer from similar problems. Several studies (for example Daprati & Gentilucci, 1997; Otto-de Haart, Carey, & Milne, 1999) used the Mu¨ller-Lyer illusion, where a stick was to be grasped that was made to look longer or shorter by placing it on arrow shaped background markings that were either inward or outward pointing (as in the standard Mu¨ller-Lyer illusion). One side effect of these background markings is that the wings of the background arrows may have unpredictable effects on the visuo-motor system. One possibility is that the wings of the inward pointing arrows (which make the stick look longer) make the system try to place the fingers cautiously within the V-shaped wings at the end. This
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may reduce the maximum grip size and give the wrong impression that grasping is based on a more accurate representation. Ironically, but we just cannot tell, the background markings might also have the opposite effect. Since the wings of inward pointing arrows extend beyond the ends of the stick they may give the visuo-motor system the signal to provide for a potentially larger object to grasp (not the stick itself but the entire object of stick and wings). This may increase grip size from what it would be if the stick were presented alone. In this case, the danger is not that the background markings produce the impression that grasping is based on a more accurate internal representation of the stickÕs length but that they cover up the existence of such an unconscious representation and make us think grasping is subject to the illusion when it is not. Indeed, many of these studies report significant grip size differences in line with the illusory effects. In fact, Franz, Fahle, Bu¨lthoff, and Gegenfurtner (2001) report stronger illusion effects on grasping the Mu¨ller-Lyer illusion than on perception. Again, if these extraneous reasons for adjusting grip size are not controlled for, we can conclude very little from these data about the existence of different representations or just a single representation for different response modalities. Several studies (for example Jackson & Shaw, 2000; Westwood & Dubrowski, 2000) used grasping in conjunction with the Ponzo illusion, where same length objects in their vertical extension were positioned at different points along four slanted horizontal lines that converged towards the centre giving the visual impression of four picture rails along a receding wall. This makes the object look longer when positioned where the lines are closer together (same retinal image projected from something seemingly more distant) than when positioned where the lines are farther apart (same retinal image projected from something seemingly closer in space). Unfortunately, the convergence of the lines also means that, depending on where the end of the object happens to be in relation to one of the lines, the line may create the tendency to narrow the grip to make it go between the line and the object. Depending on whether it happens to affect the more ‘‘distant’’ object or the ‘‘closer’’ object than it works to either enhance or, respectively, diminish the effect of the illusion in a rather uncontrolled way. Similar concerns also apply to the parallel lines illusion used by Franz et al. (2001). The important lesson from these deliberations is that, unless we can neutralise such extraneous reasons (like potential obstacles) for adjusting oneÕs action parameters (e.g., maximum grip size), then these parameters tell us little about the existence of a dual (unconscious) representation. To avoid the three problems outlined above we used new material and procedures. To avoid (at least minimize) the interference of extraneous reasons for adjusting action parameters we used the diagonal illusion as shown in Fig. 1, where the diagonal of the larger parallelogram (left) is actually shorter than the diagonal of the smaller parallelogram, but is consistently judged as longer. Participants are asked to grasp each red diagonal (as if it were a three-dimensional object) with
Fig. 1. Stimuli for the ‘‘against illusion’’ condition of Experiment 1. In the experiment the diagonals were printed in red.
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their respective hand. Important for our efforts to limit the effect of extraneous reasons on grip size is the fact that there are no obstructing background contours at either end of the diagonal, which could influence grip scaling. Of course, there are differences. For instance, the corner of the parallelogram over which the hand has to grasp is larger for the shorter diagonal (on left) than for the longer diagonal (on right). Similarly, the entire object surrounding the shorter diagonal is larger than the object surrounding the longer diagonal. Both these features could plausibly lead to greater grip sizes. However, if such an influence exists it would work in the direction of the illusion, i.e., that the shorter diagonal that looks longer would yield a larger grip size than the longer diagonal that looks shorter. Hence, if we find grip size differences in line with the diagonalsÕ true length then this cannot be attributed to these features of the surrounding figure. To counter the critique by Franz et al. (2000) of the original study by Aglioti et al. (1995) that response mode (judgment versus action) is confounded with paying attention to both displays versus only one display, we asked volunteers not only to compare the length of the two diagonals for their conscious judgment but also grasp both diagonals simultaneously. To minimize arguments about the scaling of illusion we manipulated our stimuli so that we would get a cross-over interaction between judgments of relative length and the difference between grip sizes. For that reason we made the diagonal of the larger parallelogram in the critical test condition (‘‘length differences against the illusion’’) 3 mm shorter than the other diagonal, which, nevertheless, left the shorter diagonal as being judged longer. To ensure that grasping is only based on the motor representation we avoid any delay between seeing and grasping the object. We want the participants to grasp the objects at a natural speed under as normal perceptual conditions as possible. We should also point out that Vishton, Rea, Cutting, and Nun˜ez (1999) used two-dimensional figures in their grasping condition before. Participants had no problems grasping these two-dimensional lines as if picking up a very thin object. Furthermore, as Kwok and Braddick (2003) found, grasping kinematics (grip aperture and velocity) were similar for two- and three-dimensional objects. The only difference was that the mean grip apertures for two-dimensional objects were smaller than for three-dimensional objects. Moreover, two-dimensional stimuli have certain advantages, e.g., they do not provide tactile feedback, which could lead to learned adjustment to the true size in the course of repeated grasping. In sum, we anticipate that our critical condition will show that the shorter diagonal of the larger parallelogram will be judged longer than the actually longer diagonal embedded in the smaller parallelogram, but that grip sizes will significantly be smaller for the shorter diagonal than for the longer one. In that case, we will have the most convincing evidence to date, that conscious judgment and visuo-motor action use different representations of stimulus properties.
2. Experiment 1 2.1. Method 2.1.1. Participants Sixteen volunteers were tested but due to technical problems the data of two persons could not be used. An additional person showed the strange ‘‘grasping’’ behaviour of not opening her grip at all except just before the target. Hence, there were no useable grip sizes at the point of measure-
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ment. The final sample, therefore, comprised 6 women and 7 men with a mean age of 24.9 (SD = 1.80) years. All of them but one said to be right handed (one dominant left handed and 12 dominant right handed participants). 2.1.2. Apparatus and procedure Volunteers were seated with their hands on pads on a table underneath a cover (33.8 · 14.8 cm), so that they could not see their hands for about 60% of the way when they reached forward towards the stimuli, which were placed 19.5 cm from the tableÕs edge (see Fig. 2). The hand movements were filmed with a Sony Handycam DCR-PC 1E video camera from above and forward, so that the hands were clearly visible to the camera before they became visible to the volunteers. The still pictures taken at this point (about 70% of the way from resting position to final grasp of the diagonal) in the reaching movement were analysed. Both index fingers and thumbs had a small orange sticker with a black cross on it so that the finger-thumb span at this point could be more easily measured from the video record. For this measurement the video was digitalized with a resolution of 24 pictures per second. The program ‘‘MainActor for Windows v3.6’’ extracted the single pictures of the video stream (24/s.). From this set of pictures the relevant ones were selected. Inspection of the data showed that the grasping movements did not show the typical maximum grip aperture at about 70% along the reaching trajectory. Rather, it seemed as if volunteers tended to continuously open their grip almost to the end of their movement trajectory and then quickly closed it on the target. The most likely reason, therefore, seems to be the low resolution of our camera (24 pictures/s). The mean duration for one grasping movement was about 0.5 s (SD = .12) which means that we have about 12 pictures of each grasping. As Kwok and Braddick (2003) showed, velocity of the movement is very high at the point of maximal grip aperture. From this it follows that we had too few pictures at this point (2–3). Therefore, it is very likely the participants did show a maximal grip aperture but due to our low resolution failed to detect it. Nevertheless, the pattern of grip aperture was very similar to the pattern of grip aperture towards two- and three-dimensional objects in the experiment of Kwok and Braddick (2003). Hence, we decided to measure grip size at a standard point after about 70% of the movement (when the thumbs of both hands had reached a certain reference point at 60% in the reaching
Fig. 2. Schematic representation of the experimental setup.
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movement; at this point the index fingers had reached 80% along the reaching trajectory; this means the hand as a whole had reached on average 70% of the movement.). As a consequence the average measured grip size (of the dominant hand) was only 5.88 cm even though the length of the diagonal was over 6.5 cm. The still pictures of the hand at this position were selected and transferred into the graphic program ‘‘Paint’’ in which each point of the pictures was defined by two coordinates. The grip aperture was determined by computing the norm-vector between the two reference points on thumb and index finger. For purposes of data analysis the distances between picture points used by the program were scaled back to actual distances of the environment. For grasping trials the cover was adjusted in height to prevent subjects from seeing the first 60% of their hand movements. Additionally, subjects were instructed to keep their sitting posture so that the cover just kept a green line on the table covered, which marked the 60% of their movement trajectory. They were asked to grasp the red diagonals of the two stimulus parallelograms as quickly as possible at the signal ‘‘go.’’ Between trials the stimuli were exchanged. At this point it is worth considering that measurement took place as soon as participants were able to see both— dumb and index fingers of the grasping hand. Because the movement at this point is very fast they had less than 50 ms for adjusting their grip aperture using this visual feedback. Normally, the use of visual feedback during grasping does not take place under 100 ms. (Paulignan, MacKenzie, Marteniuk, & Jeannerod, 1991). For judgment trials, volunteers were asked to indicate which of the two diagonals was the longer or whether they were the same length. If they opted for ‘‘same length’’ they were given a forced choice to decide which diagonal might be longer. 2.1.3. Materials The two plotted parallelograms were 4 cm apart. The height of the parallelogram was 4.5 cm. The angle between base and sides was 62. The base lines of the parallelograms were adjusted, so that the lengths of the diagonals were as prescribed by the three different variants shown in Table 1. Thus, the base lines of the large parallelogram were either 7.1 or 7.5 cm, the base lines of the small parallelogram varied between 2.2 and 2.6 cm. 2.1.4. Design Half the volunteers were given all judgment tasks followed by all grasping tasks while the other half of volunteers were given these tasks in the reverse order. For judgments each of the six conditions was presented once in random order. For grasping trials each left-right variant was used four times in the ‘‘with’’ and ‘‘neutral’’ condition. In the critical ‘‘against’’ condition, where actual length differences between diagonals went against the direction of the illusion, they were presented eight times each (see Table 1). They were presented in four blocks of 8 trials. In each block every left-right variation of each condition appeared once and of the critical condition twice in random order (fixed once for all participants). The order of the blocks was counterbalanced in a Latin Square design. 2.2. Results 2.2.1. Judgments Table 2 shows the number of trials in which the diagonal in the large parallelogram was judged as longer. (‘‘Spontaneous judgement’’ refers to participants judging promptly one of the diagonals
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Table 1 Stimulus presentations in Experiment 1 and 2 Length difference condition
Frequency of presentation
Against illusion
8·
Stimulus presentation and length of diagonals Experiment 1
Experiment 2
6.5
6.8
5.7
5.4
6.8
6.5
5.4
5.7
6.6
6.6
5.5
5.5
6.6
6.6
5.5
5.5
6.8
6.5
5.4
5.7
6.5
6.8
5.7
5.4
8·
Neutral
4·
4·
With illusion
4·
4·
as longer. ‘‘Forced choice’’ refers to participants judging the diagonals as being the same length and then they were asked to choose one as longer.’’) Data are collapsed for left-right presentations as there was no bias apparent. As the means make very clear, even in the ‘‘against’’ illusion condition, where the diagonal in the small parallelogram was actually 3 mm longer than the diagonal in the larger parallelogram, almost all volunteers judged the diagonal of the larger parallelogram to be longer. Significance was tested with a sign test comparing the number of volunteers who had opted more often for the larger parallelogram against those who had opted more often for the smaller parallelogram as having the longer diagonal. Table 2 Number of spontaneous judgments and under forced choice Length difference condition
Against Neutral With
Diagonal judged ‘‘longer’’
18 21 24
Sign test
Forced choice
Forced choice
5 5 2
2 0 0
1 0 0
v2
p
7.36 12.0 12.0
6.01 6.01 6.01
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2.2.2. Grasping One obvious feature was that the dominant hand behaved quite differently from the non-dominant hand. Mean grip size at the point of measurement was significantly smaller (F (1, 12) = 10.19, p < .01) for the dominant hand (6.00 cm) than for the non-dominant hand (6.45 cm). Most importantly, grip size of the non-dominant hand showed no interpretable relationship to the stimulus conditions. To illustrate, even when the actual difference in length was enhanced by the illusion (‘‘with’’ illusion condition) the average grip size was non-significantly larger (6.62 cm) for the longer diagonal than for the shorter (6.44 cm) diagonal: t (12) = 1.94, p > .05. We, therefore, analysed only the data from the dominant hand in detail. An analysis of variance was carried out on the mean grip size for the four or eight (in the ‘‘against’’ condition) trials of each condition with the factor task order (judgments first versus grasping first) between participants and factors condition (‘‘against’’ vs. ‘‘neutral’’ vs. ‘‘with’’ illusion) and parallelogram (larger vs. smaller) as within participants factors. The analysis revealed a significant interaction between parallelogram and condition (F (2, 22) = 6.93, p < .01). The relevant means in Fig. 3 show that the interaction comes about because grip size differs significantly in the direction of the true length of the diagonals. In the ‘‘with’’ illusion condition there is a just significant difference (t (12) = 2.13, p = .05). There is a practically zero difference (0.02 cm) when diagonals are the same length (t (12) = .44, p > .50). And, most importantly, grip size differs with actual length of diagonals even when it goes against the illusion (t (12) = 2.35, p < .05). Furthermore, there was a significant main effect of the factor task order (F (1, 11) = 7.98, p < .05). Subjects showed a larger grip aperture when they had to judge the length of the diagonals before they had to grasp them. No obvious interpretation of this effect comes to mind. Franz et al. (2001) pointed out another testable implication of the dual representation and the single representation models. The single representation model implies that individual differences in the susceptiblity to the illusion should be reflected in conscious judgments as well as in grasping behaviour, resulting in a positive correlation between these two measures. In contrast, the dual representation model implies that there should be no such correlation since it posits that there is no common representation subject to the illusion that influences both grasping 6,2 6,15
p = .037
p = .054
grip aperture
6,1 6,05
Parallelogram
p = .667
6
large small
5,95 5,9 5,85 5,8 5,75 against
zero Condition
with
Fig. 3. Mean grip size for dominant hand in Experiment 1.
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and judgment. To compute this correlation we computed the following susceptibility score for conscious judgments. We gave participants on each judgment trial +2 for opting spontaneously for the larger parallelogram as having the longer diagonal, +1 for doing so after forced choice, 1 for opting for the smaller parallelogram after forced choice, and 2 for doing so spontaneously. The sum of this score over all six trials was the overall susceptibility score. For grasping trials we simply computed the average difference in mean grip size between the large and the small parallelograms for each length of diagonal. Importantly, this measure is based on the means for each diagonal length and is, therefore, not biased by the fact that there were eight trials for some and only four trials for other parallelograms. The mean susceptibility score for the dominant hand in the grasping condition was 0.025 cm (not significantly different from zero: t (12) = .54, p > .50) and its correlation with the susceptibility measure for judgments was practically zero: r = .004, p = .990. Thus, there is no support for the single representation theory but evidence that grasping with the dominant hand was not influenced by the illusion in contrast to the conscious judgements. This provides some support for the dual representation model. 2.3. Discussion The results clearly show that all volunteers were strongly subject to the illusion in their conscious judgment of the diagonalsÕ relative length, even when there was an actual length difference in the opposite direction. In contrast, the results indicate that grip size scaling of the dominant hand is not (or hardly) at all influenced by the illusion. Grip size differences are significantly different in direction of the diagonalsÕ actual lengths even when the illusion works against this difference. This result shows a clear dissociation of representations of object properties in verbal judgment and action. The fact that judgment and action dissociated with our stimulus material and procedure is important for two reasons. (1) The material is (unlike the Ebbinghaus/Titchener circles and other traditional illusions) free of ÔvisualÕ obstacles for the grasping movement. This makes it difficult for the single representation model to defend the use of only one representation by claiming that the grip size differences deviate from the conscious judgments due to extraneous reasons for adjusting grip size (e.g., having to squeeze in fingers, etc.). (2) We tried to equate the grasping and the judgment conditions for the need to consider both illusory stimuli simultaneously, by letting volunteers grasp both diagonals at the same time. This attempt, however, partly failed. The larger grip sizes of the non-dominant hand and its insensitivity to the actual length of the diagonals or illusion effects alike suggests that despite the need for grasping both diagonals volunteers only concentrated on their dominant hand. To improve on this deficiency we made a renewed effort to equate grasping and conscious judgment conditions in our next experiment.
3. Experiment 2 One remaining problem in Experiment 1 was that instructions to grasp two objects simultaneously do not ensure the same degree of simultaneous attention to both objects as the
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instructions to compare the length of the two objects. To improve the comparability of attentional factors across the grasping and the judgment conditions we asked volunteers to provide conscious length judgments for each diagonal by indicating the estimated length with their finger-thumb span as used previously by Haffenden and Goodale (1998); Haffenden et al., 2001. Unlike a direct length comparison of two diagonals this measure provides independent length estimates in the same way as simultaneous grasping. Franz (2001) has argued, though, that manual estimates are a non-standard measure of perception, which tends to inflate illusory differences. This inflation, however, cannot be due to a difference in the illusory effect, i.e., the mapping from stimulus to internal representation. It must occur in how the internal representation is mapped onto the observable response. This kind of inflation, fortunately, cannot produce the interaction we seek in the critical ‘‘against illusion’’ condition, i.e., that grip scaling during grasping indicates a difference in the direction of diagonalsÕ actual lengths while length estimates given by finger-thumb spans yield a difference in the opposite direction. We used the opportunity of a new study to introduce a slight modification of our stimulus material, by making that part of each stimulus display, which is in the trajectory of the approaching hand, identical for both displays as shown in Fig. 4. 3.1. Method 3.1.1. Participants Nineteen participants were tested but due to a technical problem the data of one person could not be used. The final sample, therefore, comprised 11 women and 7 men, with a mean age of 24.83 (SD = 3.07) years. Thirteen participants said to be dominant right handed, five said to be dominant left handed. In every case participantsÕ self-classification was confirmed by the HDT handedness test (Steingru¨ber & Lienert, 1976). 3.1.2. Apparatus and procedure The procedure for the grasping condition was the same as in the first experiment with the exception of the use of the finger-thumb span for indicating consciously perceived length differences. Participants were seated, like in the grasping condition, with their hands on pads underneath a cover (see Fig. 2). However, the pads were moved 10 cm away from table edge, which was necessary for recording the hands on video at the same point between body and stimulus as in the grasping condition. Also, the cover was raised to make sure that the participants could not see their hands while indicating the length of the diagonals. As in the grasping condition, participants were asked to match the red diagonals with their index fingers and thumbs as quickly as possible at the signal ‘‘go.’’ They were instructed to do this simultaneously with both hands. Pictures for measurement were selected when participants held their grip aperture steady for about one second. Otherwise this judgment condition was exactly the same as the grasping condition. 3.1.3. Materials The two plotted stimulus figures (see Fig. 4) were 4.5 cm apart. The base lines of the figures varied between 4.0 and 4.2 cm, the height varied between 3.6 and 3.9 cm so that the lengths of the diagonals were as prescribed by three different variants shown in Table 1. The angle of the
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Fig. 4. Stimuli for the ‘‘against illusion’’ condition of Experiment 2. In the experiment the diagonals were printed in red.
diagonals in both figures was kept constant at exactly 43 tilted towards the grasping hand. 3.1.4. Design The design was exactly the same as in Experiment 1 except that the manual judgment condition replaced the verbal comparative length judgments. This made it possible to copy the design of the grasping condition for the judgment condition. 3.2. Results 3.2.1. Indicating length (judgments) We analysed both hands separately to make a direct comparison with the grasping condition possible. For the dominant hand an analysis of variance was carried out on the mean fingerthumb aperture over the four or eight (in the ‘‘against condition’’) trials of each condition with the factor task order (matching first versus grasping first) between participants and factors condition (‘‘against’’ vs. ‘‘neutral’’ vs. ‘‘with illusion’’) and figure (larger vs. smaller) as within participants factors. The analysis revealed a significant main effect for factor figure (F (1, 16) = 48.42, p < .001). Finger-thumb aperture was larger for the larger figure in all of the three conditions. Furthermore, there was a significant interaction between figure and condition (F (2, 32) = 5.47, p < .01). Fig. 5A shows that the interaction comes about because finger-thumb aperture depended on actual size difference (condition) and effects of the illusion (figure). In fact the influence of the illusion was so strong that even in the ‘‘against’’ condition it overpowered the actual length difference in the opposite direction: t (17) = 2.11, p = .05. The same analysis of variance for the non-dominant hand showed the same effects for figure (F (1, 16) = 18.49, p < .01) and the interaction between figure and condition (F (2, 32) = 5.67, p < .01) significant. As Fig. 5B shows the means look very similar to those from the dominant hand. However, the illusion was not strong enough make a significant difference in the ‘‘against’’ condition (t(17) = .73, p = .48), only in the ‘‘neutral’’ condition (t(17) = 3.35, p < .01). 3.2.2. Grasping Again, the grasping movements did not show the typical maximum grip aperture. For this reason, we analysed the pictures at the point where the hands had gone through about 70% of their trajectory. As in the first experiment, the dominant hand behaved quite differently from the non-dominant hand. Although there was only a very small difference in grip size at the point
B
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Fig. 5. (A) Mean finger-thumb aperture as indicator of perceived length in Experiment: dominant hand. (B) Mean finger-thumb aperture as indicator of perceived length in Experiment: non-dominant hand
of measurement between dominant hand (5.62 cm) and non-dominant hand (5.50 cm), the grip size of the non-dominant hand showed again no interpretable relationship with the stimulus conditions. To illustrate, even when the actually longer diagonal was made to look even longer in the ‘‘with’’ illusion condition was the average grip size was smaller (5.67 cm) than for the shorter diagonal (5.80 cm), though non-significantly so: t (17) = 1.49, p > .05. We, therefore, only analysed the data from the dominant hand in detail. The same analysis of variance was carried out as on the manual judgment data described above. The only significant effect was the interaction between figure and condition (F (2, 30) = 11.51, p < .001). The relevant means in Fig. 6 show that the interaction comes about because grip size differs significantly in the direction of the true length of the diagonals. In the ‘‘with’’ illusion condition there is a highly significant difference (t (17) = 5.37, p < .001). There is practically no difference when diagonals are the same length (t (16) = .66, p > .50). And, most importantly, grip size differs with actual length of diagonals even when it goes against the illusion (t (17) = 3.31, p < .01).
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grip aperture
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Fig. 6. Mean grip size for dominant hand in Experiment 2.
3.2.3. Susceptibility to the illusion We also computed scores for the susceptibility to the illusion in the grasping and in the judgment condition. For grasping trials we used the same measure as in Experiment 1 and for judgment trials we computed the corresponding measure as for grasping. For the dominant hand the Pearson correlation coefficient for these two susceptibility measures was non-significantly positive: r = .10, p = .71. This provides yet again no evidence for the single representation model. Moreover, the mean susceptibility score in the grasping condition was .032 cm which is not significantly different from zero (t (16) = .81, p = .43).1 In contrast, the mean susceptibility score for the judgment condition was .55 cm which is highly significantly different from zero (t (17) = 7.21, p < .001). Comparison of the two scores makes clear that susceptibility to illusion was much greater for judgments than for grasping: t (16) = 6.04, p < .001. A similar result emerged for judgement by the non-dominant hand. The susceptibility score of 0.46 cm was highly significant (t (17) = 3.71, p < .002). 3.3. Discussion The grasping data closely replicate the data from Experiment 1. Again grip size scaling on the non-dominant hand was insensitive to the stimulus differences (even more so than in Experiment 1), while the dominant hand behaved much more disciplined and reflected stimulus differences minutely. There was no indication that the illusion affected the dominant handÕs grip scaling. In stark contrast, when finger-thumb scaling was used as conscious estimates of stimulus lengths, estimates always went significantly in the direction of the illusion even in the ‘‘against illusion’’ condition, where the actual lengths differed in the opposite direction. Knowing from Experiment 1 that despite the need for bimanual grasping the non-dominant hand showed such imprecise grip size scaling, we expected a similar finding for the finger-thumb 1
There is a change in the degrees of freedom because data of one participant are incomplete.
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scaling as indicators of perceived size. To our surprise, in this task the non-dominant hand produced results comparable to the dominant hand, showing comparable sensitivity to stimulus differences, though the influence of the illusion was not as strong. This indicates that, despite the apparent parallelism in task demands between grasping and conscious judgment by thumb-finger span, there still remained some attentional asymmetry between the grasping and judgment task.
4. General discussion The main objective in this study was to find illusory stimuli, which do not require visual ‘‘obstacles’’ in the path of the grasping hand, to avoid extraneous reasons for grip size scaling apart from the length of the to be grasped object. Such extraneous reasons can have potential effects that can be exploited by opposing interpretations. As Jacob and Jeannerod (2003) point out, they detract from the interpretation that grasping is less influenced by illusions than conscious judgment, because the observed difference could be due to such extraneous reasons rather than a difference in internal representation. Such visual obstacles may, however, also lead to the opposite effect of changing the grip scaling in the direction of the illusion. The data from Haffenden and Goodale (1998, 2000) provide concrete evidence for this danger in the Ebbinghaus/Titchener illusion. But also the Mu¨ller-Lyer illusion may be particularly prone to this, because the arrow wings at the end of the stick may be perceived by the motor system as belonging to the object to be grasped. Consequently, the grip size will be larger for these objects but for a reason different from the illusory effect created by these arrow wings. In sum, because such extraneous reasons were avoided in our stimuli our results provide improved support for the dual representation theory. There is little indication that grip size scaling of the dominant hand is subject to the illusion (and the non-dominant hand showed some signs of being influenced only in Experiment 1). In contrast, comparisons judgments in Experiment 1 and manual estimates on both hands in Experiment 2 as measures of conscious perception showed very strong effects of the illusion.
References Aglioti, S., DeSouza, J. F., & Goodale, M. A. (1995). Size-contrast illusions deceive the eye but not the hand. Current Biology, 5(6), 679–685. Bridgeman, B., Gemmer, A., Forsman, T., & Huemer, V. (2000). Processing spatial information in the sensorimotor branch of the visual system. Vision Research, 40, 3539–3552. Bridgeman, B., Kirch, M., & Sperling, A. (1981). Segregation of cognitive and motor aspects of visual function using induced motion. Perception & Psychophysics, 29, 336–342. Bridgeman, B., Peery, S., & Anand, S. (1997). Interaction of cognitive and sensorimotor maps of visual space. Perception & Psychophysics, 59(3), 456–469. Daprati, E., & Gentilucci, M. (1997). Grasping an illusion. Neuropsychologica, 35, 1577–1582. Franz, V. H., Bu¨lthoff, H. H., & Fahle, M. (2003). Grasp effects of the Ebbinhaus illusion: Obstacle avoidance is not the explanation. Experimental brain research, 149, 470–477. Franz, V. H., Gegenfurtner, K. R., Bu¨lthoff, H. H., & Fahle, M. (2000). Grasping visual illusions: No evidence for a dissociation between perception and action. Psychological Science, 11, 20–25.
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Dissociating size representation for action and for conscious judgment: Grasping visual illusions without apparent obstacles Elisabeth Sto¨ttinger, Josef Perner * Department of Psychology, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria Received 12 May 2005 Available online 9 September 2005
Abstract Visual illusions provide important evidence for the co-existence of unconscious and conscious representations. Objects surrounded by other figures (e.g., a disc surrounded by smaller or larger rings, Ebbinghaus/ Titchener illusion) are consciously perceived as different in size, while the visuo-motor system supposedly uses an unconscious representation of the discsÕ true size for grip size scaling. Recent evidence suggests other factors than represented size, e.g., surrounding rings conceived as obstacles, affect grip size. Use of the diagonal illusion avoids visual obstacles in the path of the reaching hand. Results support the dual representation theory. Grip size scaling follows actual size independent of illusory effects, which clearly bias conscious perception in direct comparisons of lengths (Experiment 1) and in finger-thumb span indications of perceived length (Experiment 2). 2005 Elsevier Inc. All rights reserved. Keywords: Visual illusions; Perceptual judgement; Knowledge for action; Implicit knowledge; Conscious perception; Obstacles avoidance
1. Introduction The theory that it is possible to have visually gained knowledge without conscious awareness has been greatly helped by the finding from blindsight patients (e.g., Perenin & Rossetti, 1996; *
Corresponding author. E-mail address: [email protected] (J. Perner).
1053-8100/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.concog.2005.07.004
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Po¨ppel, Held, & Frost, 1973; Weiskrantz, Sanders, & Marshall, 1974; Weiskrantz, 1998). Despite complete lack of conscious perception of stimuli in the ÔblindÕ part of their visual field they can, nevertheless, fairly accurately point to the stimulus or accurately act in other ways in relation to it. Support for the existence of similar dissociations in neurologically healthy people comes from experiments with illusory stimuli. Early demonstrations relied on motion illusions using the induced Roelofs Effect investigating eye gaze (Wong & Mack, 1981) and manual action (Bridgeman, Gemmer, Forsman, & Huemer, 2000; Bridgeman, Kirch, & Sperling, 1981). More recently, Aglioti, DeSouza, and Goodale (1995) used the Ebbinghaus/Titchener circles illusion to show a corresponding dissociation. When asked to indicate which of two discs of the same size was larger, the one surrounded by smaller rings was judged larger than the one surrounded by larger rings— in keeping with the known effect of this illusion. However, the grip size scaling (thumb finger opening on the way to grasping a disc) was about the same, indicating that the sensory-motor system allegedly uses a different—more accurate—representation (knowledge) of the discsÕ true sizes than conscious perception. This interpretation of the original finding by Aglioti et al. (1995) has, however, come in for much criticism. We concentrate here on the question whether the results of this experiment or any of its later variations requires for their explanation the existence of two contradictory representations of reality (dual representations model)—one of which is available to conscious awareness and the other one is not—or whether they can be explained on the basis of a single, conscious representation (single representation model). For answering this question the following aspects of the experimental setup and model of relevant mental processes are critical. In the basic setup a particular stimulus object is presented and one of its properties (e.g., its size) is assessed by two different response modes (e.g., a verbal judgment and a grasping action) which yield contradictory response mode information about the stimulus property. The dual representations model assumes that the contradictory information in the different response modes comes from different representations. However, the single representation model can also explain this contradictory response mode information if one of the following two possibilities arises: 1. The mapping from stimulus to internal representation is not constant over the two tasks in which each response mode is used. Pavani, Boscagli, Benvenuti, Rabuffetti, and Farne` (1999) and, in particular, Franz, Gegenfurtner, Bu¨lthoff, and Fahle (2000) have capitalized on this possibility by observing that the size of the illusory effect is much stronger when the two discs in each display, one disc surrounded by smaller the other disc by larger rings, are directly compared than when a disc in one of these displays is compared to a plain disc (without any surrounding rings). Participants in AgliottiÕs experiment had to judge size by comparing both displays, resulting in relatively large differences between represented and actual size. In contrast, participants had to grasp only one of the two discs, which might have restricted their attention to only that particular disc, attenuating the illusory effect and, consequently, resulting in seemingly more accurate grip size differences. Hence, on each occasion only a single representation of each discÕs size might be formed, but due to the attentional differences (attend to both displays for judgment but only to one when grasping) that representation is more accurate in the grasping than in the judgment task. Consequently, the single representation theory can adequately explain the difference in manually and verbally expressed size. 2. Extraneous reasons for acting may bias the action response. When asked to give a conscious judgment of the size of a disc then the best estimate available is the size as internally represented.
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In contrast, when grasping the disc there may be different valid reasons for a particular fingerthumb span (Jacob & Jeannerod, 2003; Smeets & Brenner, 1999). The actual size of the disc will only be one of them. Another good reason is the space between the disc and surrounding obstacles. If the disc is presented on its own, one can afford a fairly large maximum grip span that leisurely narrows in on the true size of the disc as the hand approaches. If, however, there is barely space to fit oneÕs fingers between the disc and surrounding objects then one will adjust oneÕs grip size to a minimum above the size of the disc. In fact, Haffenden and Goodale (2000) and Haffenden, Schiff, and Goodale (2001) found that the visuo-motor system seems to make exactly such adjustments—even when the surrounding obstacles are only two-dimensional marks on the supporting surface (and strictly speaking are not real obstacles). Haffenden and Goodale (2000) used an additional ‘‘new’’ Ebbinghaus figure where the gap between target and surrounding small circles was the same as between target and surrounding large circles. They found that the critical variable for grasping was not the perceived size of the target but the distance between target and context circles. However, Franz, Bu¨lthoff, and Fahle (2003) did a similar experiment with another additional Ebbinghaus figure (the same gap between target and surrounding large circles as between target and surrounding small circles). They found a systematically increasing grip aperture with perceived size—independent of the gap between target and context circles. In their experiment participants had to grasp the target circle of the four context conditions with their dominant hand. As soon as participants started their grasping movements, shutter glasses suppressed their vision preventing any kind of visual feedback throughout the entire movement. Participants had four seconds to carry out this movement, which creates a problem of interpretation pointed out by Millner and Goodale (1995): the unconscious representation for guiding movements does not persist for longer than 2 s. If the delay between seeing an object and movement execution toward this object is longer than 2 s, movements are based on the longer lasting conscious perceptual representation, as found by Goodale, Jakobson, and Keillor (1994) with the visual form agnosia patient D.F. See also Bridgeman, Peery, and Anand (1997), Marcel (1993). Hence, if participants in the experiment by Franz et al. (2003) did take close to 2 s for their movement with shutter glasses, it is fairly likely that these authors found an influence of the illusion on grasping because the motor representation had decayed and participants were forced to base their grasping on the illusory perceptual representation. In any case, the Ebbinghaus Illusion is an inadequate method for deciding whether grasp apertures depends on the size of the objects to be grasped, because the object is surrounded by context circles (obstacles) and the gap between target and small and large context circles can never be matched completely. It remains an irresolvable question as to how to control the distance between target disc and context circles: is it to be controlled at the narrowest point or in terms of the area between two context circles and the target object, etc. Other illusory stimuli that have been used in this area suffer from similar problems. Several studies (for example Daprati & Gentilucci, 1997; Otto-de Haart, Carey, & Milne, 1999) used the Mu¨ller-Lyer illusion, where a stick was to be grasped that was made to look longer or shorter by placing it on arrow shaped background markings that were either inward or outward pointing (as in the standard Mu¨ller-Lyer illusion). One side effect of these background markings is that the wings of the background arrows may have unpredictable effects on the visuo-motor system. One possibility is that the wings of the inward pointing arrows (which make the stick look longer) make the system try to place the fingers cautiously within the V-shaped wings at the end. This
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may reduce the maximum grip size and give the wrong impression that grasping is based on a more accurate representation. Ironically, but we just cannot tell, the background markings might also have the opposite effect. Since the wings of inward pointing arrows extend beyond the ends of the stick they may give the visuo-motor system the signal to provide for a potentially larger object to grasp (not the stick itself but the entire object of stick and wings). This may increase grip size from what it would be if the stick were presented alone. In this case, the danger is not that the background markings produce the impression that grasping is based on a more accurate internal representation of the stickÕs length but that they cover up the existence of such an unconscious representation and make us think grasping is subject to the illusion when it is not. Indeed, many of these studies report significant grip size differences in line with the illusory effects. In fact, Franz, Fahle, Bu¨lthoff, and Gegenfurtner (2001) report stronger illusion effects on grasping the Mu¨ller-Lyer illusion than on perception. Again, if these extraneous reasons for adjusting grip size are not controlled for, we can conclude very little from these data about the existence of different representations or just a single representation for different response modalities. Several studies (for example Jackson & Shaw, 2000; Westwood & Dubrowski, 2000) used grasping in conjunction with the Ponzo illusion, where same length objects in their vertical extension were positioned at different points along four slanted horizontal lines that converged towards the centre giving the visual impression of four picture rails along a receding wall. This makes the object look longer when positioned where the lines are closer together (same retinal image projected from something seemingly more distant) than when positioned where the lines are farther apart (same retinal image projected from something seemingly closer in space). Unfortunately, the convergence of the lines also means that, depending on where the end of the object happens to be in relation to one of the lines, the line may create the tendency to narrow the grip to make it go between the line and the object. Depending on whether it happens to affect the more ‘‘distant’’ object or the ‘‘closer’’ object than it works to either enhance or, respectively, diminish the effect of the illusion in a rather uncontrolled way. Similar concerns also apply to the parallel lines illusion used by Franz et al. (2001). The important lesson from these deliberations is that, unless we can neutralise such extraneous reasons (like potential obstacles) for adjusting oneÕs action parameters (e.g., maximum grip size), then these parameters tell us little about the existence of a dual (unconscious) representation. To avoid the three problems outlined above we used new material and procedures. To avoid (at least minimize) the interference of extraneous reasons for adjusting action parameters we used the diagonal illusion as shown in Fig. 1, where the diagonal of the larger parallelogram (left) is actually shorter than the diagonal of the smaller parallelogram, but is consistently judged as longer. Participants are asked to grasp each red diagonal (as if it were a three-dimensional object) with
Fig. 1. Stimuli for the ‘‘against illusion’’ condition of Experiment 1. In the experiment the diagonals were printed in red.
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their respective hand. Important for our efforts to limit the effect of extraneous reasons on grip size is the fact that there are no obstructing background contours at either end of the diagonal, which could influence grip scaling. Of course, there are differences. For instance, the corner of the parallelogram over which the hand has to grasp is larger for the shorter diagonal (on left) than for the longer diagonal (on right). Similarly, the entire object surrounding the shorter diagonal is larger than the object surrounding the longer diagonal. Both these features could plausibly lead to greater grip sizes. However, if such an influence exists it would work in the direction of the illusion, i.e., that the shorter diagonal that looks longer would yield a larger grip size than the longer diagonal that looks shorter. Hence, if we find grip size differences in line with the diagonalsÕ true length then this cannot be attributed to these features of the surrounding figure. To counter the critique by Franz et al. (2000) of the original study by Aglioti et al. (1995) that response mode (judgment versus action) is confounded with paying attention to both displays versus only one display, we asked volunteers not only to compare the length of the two diagonals for their conscious judgment but also grasp both diagonals simultaneously. To minimize arguments about the scaling of illusion we manipulated our stimuli so that we would get a cross-over interaction between judgments of relative length and the difference between grip sizes. For that reason we made the diagonal of the larger parallelogram in the critical test condition (‘‘length differences against the illusion’’) 3 mm shorter than the other diagonal, which, nevertheless, left the shorter diagonal as being judged longer. To ensure that grasping is only based on the motor representation we avoid any delay between seeing and grasping the object. We want the participants to grasp the objects at a natural speed under as normal perceptual conditions as possible. We should also point out that Vishton, Rea, Cutting, and Nun˜ez (1999) used two-dimensional figures in their grasping condition before. Participants had no problems grasping these two-dimensional lines as if picking up a very thin object. Furthermore, as Kwok and Braddick (2003) found, grasping kinematics (grip aperture and velocity) were similar for two- and three-dimensional objects. The only difference was that the mean grip apertures for two-dimensional objects were smaller than for three-dimensional objects. Moreover, two-dimensional stimuli have certain advantages, e.g., they do not provide tactile feedback, which could lead to learned adjustment to the true size in the course of repeated grasping. In sum, we anticipate that our critical condition will show that the shorter diagonal of the larger parallelogram will be judged longer than the actually longer diagonal embedded in the smaller parallelogram, but that grip sizes will significantly be smaller for the shorter diagonal than for the longer one. In that case, we will have the most convincing evidence to date, that conscious judgment and visuo-motor action use different representations of stimulus properties.
2. Experiment 1 2.1. Method 2.1.1. Participants Sixteen volunteers were tested but due to technical problems the data of two persons could not be used. An additional person showed the strange ‘‘grasping’’ behaviour of not opening her grip at all except just before the target. Hence, there were no useable grip sizes at the point of measure-
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ment. The final sample, therefore, comprised 6 women and 7 men with a mean age of 24.9 (SD = 1.80) years. All of them but one said to be right handed (one dominant left handed and 12 dominant right handed participants). 2.1.2. Apparatus and procedure Volunteers were seated with their hands on pads on a table underneath a cover (33.8 · 14.8 cm), so that they could not see their hands for about 60% of the way when they reached forward towards the stimuli, which were placed 19.5 cm from the tableÕs edge (see Fig. 2). The hand movements were filmed with a Sony Handycam DCR-PC 1E video camera from above and forward, so that the hands were clearly visible to the camera before they became visible to the volunteers. The still pictures taken at this point (about 70% of the way from resting position to final grasp of the diagonal) in the reaching movement were analysed. Both index fingers and thumbs had a small orange sticker with a black cross on it so that the finger-thumb span at this point could be more easily measured from the video record. For this measurement the video was digitalized with a resolution of 24 pictures per second. The program ‘‘MainActor for Windows v3.6’’ extracted the single pictures of the video stream (24/s.). From this set of pictures the relevant ones were selected. Inspection of the data showed that the grasping movements did not show the typical maximum grip aperture at about 70% along the reaching trajectory. Rather, it seemed as if volunteers tended to continuously open their grip almost to the end of their movement trajectory and then quickly closed it on the target. The most likely reason, therefore, seems to be the low resolution of our camera (24 pictures/s). The mean duration for one grasping movement was about 0.5 s (SD = .12) which means that we have about 12 pictures of each grasping. As Kwok and Braddick (2003) showed, velocity of the movement is very high at the point of maximal grip aperture. From this it follows that we had too few pictures at this point (2–3). Therefore, it is very likely the participants did show a maximal grip aperture but due to our low resolution failed to detect it. Nevertheless, the pattern of grip aperture was very similar to the pattern of grip aperture towards two- and three-dimensional objects in the experiment of Kwok and Braddick (2003). Hence, we decided to measure grip size at a standard point after about 70% of the movement (when the thumbs of both hands had reached a certain reference point at 60% in the reaching
Fig. 2. Schematic representation of the experimental setup.
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movement; at this point the index fingers had reached 80% along the reaching trajectory; this means the hand as a whole had reached on average 70% of the movement.). As a consequence the average measured grip size (of the dominant hand) was only 5.88 cm even though the length of the diagonal was over 6.5 cm. The still pictures of the hand at this position were selected and transferred into the graphic program ‘‘Paint’’ in which each point of the pictures was defined by two coordinates. The grip aperture was determined by computing the norm-vector between the two reference points on thumb and index finger. For purposes of data analysis the distances between picture points used by the program were scaled back to actual distances of the environment. For grasping trials the cover was adjusted in height to prevent subjects from seeing the first 60% of their hand movements. Additionally, subjects were instructed to keep their sitting posture so that the cover just kept a green line on the table covered, which marked the 60% of their movement trajectory. They were asked to grasp the red diagonals of the two stimulus parallelograms as quickly as possible at the signal ‘‘go.’’ Between trials the stimuli were exchanged. At this point it is worth considering that measurement took place as soon as participants were able to see both— dumb and index fingers of the grasping hand. Because the movement at this point is very fast they had less than 50 ms for adjusting their grip aperture using this visual feedback. Normally, the use of visual feedback during grasping does not take place under 100 ms. (Paulignan, MacKenzie, Marteniuk, & Jeannerod, 1991). For judgment trials, volunteers were asked to indicate which of the two diagonals was the longer or whether they were the same length. If they opted for ‘‘same length’’ they were given a forced choice to decide which diagonal might be longer. 2.1.3. Materials The two plotted parallelograms were 4 cm apart. The height of the parallelogram was 4.5 cm. The angle between base and sides was 62. The base lines of the parallelograms were adjusted, so that the lengths of the diagonals were as prescribed by the three different variants shown in Table 1. Thus, the base lines of the large parallelogram were either 7.1 or 7.5 cm, the base lines of the small parallelogram varied between 2.2 and 2.6 cm. 2.1.4. Design Half the volunteers were given all judgment tasks followed by all grasping tasks while the other half of volunteers were given these tasks in the reverse order. For judgments each of the six conditions was presented once in random order. For grasping trials each left-right variant was used four times in the ‘‘with’’ and ‘‘neutral’’ condition. In the critical ‘‘against’’ condition, where actual length differences between diagonals went against the direction of the illusion, they were presented eight times each (see Table 1). They were presented in four blocks of 8 trials. In each block every left-right variation of each condition appeared once and of the critical condition twice in random order (fixed once for all participants). The order of the blocks was counterbalanced in a Latin Square design. 2.2. Results 2.2.1. Judgments Table 2 shows the number of trials in which the diagonal in the large parallelogram was judged as longer. (‘‘Spontaneous judgement’’ refers to participants judging promptly one of the diagonals
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Table 1 Stimulus presentations in Experiment 1 and 2 Length difference condition
Frequency of presentation
Against illusion
8·
Stimulus presentation and length of diagonals Experiment 1
Experiment 2
6.5
6.8
5.7
5.4
6.8
6.5
5.4
5.7
6.6
6.6
5.5
5.5
6.6
6.6
5.5
5.5
6.8
6.5
5.4
5.7
6.5
6.8
5.7
5.4
8·
Neutral
4·
4·
With illusion
4·
4·
as longer. ‘‘Forced choice’’ refers to participants judging the diagonals as being the same length and then they were asked to choose one as longer.’’) Data are collapsed for left-right presentations as there was no bias apparent. As the means make very clear, even in the ‘‘against’’ illusion condition, where the diagonal in the small parallelogram was actually 3 mm longer than the diagonal in the larger parallelogram, almost all volunteers judged the diagonal of the larger parallelogram to be longer. Significance was tested with a sign test comparing the number of volunteers who had opted more often for the larger parallelogram against those who had opted more often for the smaller parallelogram as having the longer diagonal. Table 2 Number of spontaneous judgments and under forced choice Length difference condition
Against Neutral With
Diagonal judged ‘‘longer’’
18 21 24
Sign test
Forced choice
Forced choice
5 5 2
2 0 0
1 0 0
v2
p
7.36 12.0 12.0
6.01 6.01 6.01
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2.2.2. Grasping One obvious feature was that the dominant hand behaved quite differently from the non-dominant hand. Mean grip size at the point of measurement was significantly smaller (F (1, 12) = 10.19, p < .01) for the dominant hand (6.00 cm) than for the non-dominant hand (6.45 cm). Most importantly, grip size of the non-dominant hand showed no interpretable relationship to the stimulus conditions. To illustrate, even when the actual difference in length was enhanced by the illusion (‘‘with’’ illusion condition) the average grip size was non-significantly larger (6.62 cm) for the longer diagonal than for the shorter (6.44 cm) diagonal: t (12) = 1.94, p > .05. We, therefore, analysed only the data from the dominant hand in detail. An analysis of variance was carried out on the mean grip size for the four or eight (in the ‘‘against’’ condition) trials of each condition with the factor task order (judgments first versus grasping first) between participants and factors condition (‘‘against’’ vs. ‘‘neutral’’ vs. ‘‘with’’ illusion) and parallelogram (larger vs. smaller) as within participants factors. The analysis revealed a significant interaction between parallelogram and condition (F (2, 22) = 6.93, p < .01). The relevant means in Fig. 3 show that the interaction comes about because grip size differs significantly in the direction of the true length of the diagonals. In the ‘‘with’’ illusion condition there is a just significant difference (t (12) = 2.13, p = .05). There is a practically zero difference (0.02 cm) when diagonals are the same length (t (12) = .44, p > .50). And, most importantly, grip size differs with actual length of diagonals even when it goes against the illusion (t (12) = 2.35, p < .05). Furthermore, there was a significant main effect of the factor task order (F (1, 11) = 7.98, p < .05). Subjects showed a larger grip aperture when they had to judge the length of the diagonals before they had to grasp them. No obvious interpretation of this effect comes to mind. Franz et al. (2001) pointed out another testable implication of the dual representation and the single representation models. The single representation model implies that individual differences in the susceptiblity to the illusion should be reflected in conscious judgments as well as in grasping behaviour, resulting in a positive correlation between these two measures. In contrast, the dual representation model implies that there should be no such correlation since it posits that there is no common representation subject to the illusion that influences both grasping 6,2 6,15
p = .037
p = .054
grip aperture
6,1 6,05
Parallelogram
p = .667
6
large small
5,95 5,9 5,85 5,8 5,75 against
zero Condition
with
Fig. 3. Mean grip size for dominant hand in Experiment 1.
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and judgment. To compute this correlation we computed the following susceptibility score for conscious judgments. We gave participants on each judgment trial +2 for opting spontaneously for the larger parallelogram as having the longer diagonal, +1 for doing so after forced choice, 1 for opting for the smaller parallelogram after forced choice, and 2 for doing so spontaneously. The sum of this score over all six trials was the overall susceptibility score. For grasping trials we simply computed the average difference in mean grip size between the large and the small parallelograms for each length of diagonal. Importantly, this measure is based on the means for each diagonal length and is, therefore, not biased by the fact that there were eight trials for some and only four trials for other parallelograms. The mean susceptibility score for the dominant hand in the grasping condition was 0.025 cm (not significantly different from zero: t (12) = .54, p > .50) and its correlation with the susceptibility measure for judgments was practically zero: r = .004, p = .990. Thus, there is no support for the single representation theory but evidence that grasping with the dominant hand was not influenced by the illusion in contrast to the conscious judgements. This provides some support for the dual representation model. 2.3. Discussion The results clearly show that all volunteers were strongly subject to the illusion in their conscious judgment of the diagonalsÕ relative length, even when there was an actual length difference in the opposite direction. In contrast, the results indicate that grip size scaling of the dominant hand is not (or hardly) at all influenced by the illusion. Grip size differences are significantly different in direction of the diagonalsÕ actual lengths even when the illusion works against this difference. This result shows a clear dissociation of representations of object properties in verbal judgment and action. The fact that judgment and action dissociated with our stimulus material and procedure is important for two reasons. (1) The material is (unlike the Ebbinghaus/Titchener circles and other traditional illusions) free of ÔvisualÕ obstacles for the grasping movement. This makes it difficult for the single representation model to defend the use of only one representation by claiming that the grip size differences deviate from the conscious judgments due to extraneous reasons for adjusting grip size (e.g., having to squeeze in fingers, etc.). (2) We tried to equate the grasping and the judgment conditions for the need to consider both illusory stimuli simultaneously, by letting volunteers grasp both diagonals at the same time. This attempt, however, partly failed. The larger grip sizes of the non-dominant hand and its insensitivity to the actual length of the diagonals or illusion effects alike suggests that despite the need for grasping both diagonals volunteers only concentrated on their dominant hand. To improve on this deficiency we made a renewed effort to equate grasping and conscious judgment conditions in our next experiment.
3. Experiment 2 One remaining problem in Experiment 1 was that instructions to grasp two objects simultaneously do not ensure the same degree of simultaneous attention to both objects as the
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instructions to compare the length of the two objects. To improve the comparability of attentional factors across the grasping and the judgment conditions we asked volunteers to provide conscious length judgments for each diagonal by indicating the estimated length with their finger-thumb span as used previously by Haffenden and Goodale (1998); Haffenden et al., 2001. Unlike a direct length comparison of two diagonals this measure provides independent length estimates in the same way as simultaneous grasping. Franz (2001) has argued, though, that manual estimates are a non-standard measure of perception, which tends to inflate illusory differences. This inflation, however, cannot be due to a difference in the illusory effect, i.e., the mapping from stimulus to internal representation. It must occur in how the internal representation is mapped onto the observable response. This kind of inflation, fortunately, cannot produce the interaction we seek in the critical ‘‘against illusion’’ condition, i.e., that grip scaling during grasping indicates a difference in the direction of diagonalsÕ actual lengths while length estimates given by finger-thumb spans yield a difference in the opposite direction. We used the opportunity of a new study to introduce a slight modification of our stimulus material, by making that part of each stimulus display, which is in the trajectory of the approaching hand, identical for both displays as shown in Fig. 4. 3.1. Method 3.1.1. Participants Nineteen participants were tested but due to a technical problem the data of one person could not be used. The final sample, therefore, comprised 11 women and 7 men, with a mean age of 24.83 (SD = 3.07) years. Thirteen participants said to be dominant right handed, five said to be dominant left handed. In every case participantsÕ self-classification was confirmed by the HDT handedness test (Steingru¨ber & Lienert, 1976). 3.1.2. Apparatus and procedure The procedure for the grasping condition was the same as in the first experiment with the exception of the use of the finger-thumb span for indicating consciously perceived length differences. Participants were seated, like in the grasping condition, with their hands on pads underneath a cover (see Fig. 2). However, the pads were moved 10 cm away from table edge, which was necessary for recording the hands on video at the same point between body and stimulus as in the grasping condition. Also, the cover was raised to make sure that the participants could not see their hands while indicating the length of the diagonals. As in the grasping condition, participants were asked to match the red diagonals with their index fingers and thumbs as quickly as possible at the signal ‘‘go.’’ They were instructed to do this simultaneously with both hands. Pictures for measurement were selected when participants held their grip aperture steady for about one second. Otherwise this judgment condition was exactly the same as the grasping condition. 3.1.3. Materials The two plotted stimulus figures (see Fig. 4) were 4.5 cm apart. The base lines of the figures varied between 4.0 and 4.2 cm, the height varied between 3.6 and 3.9 cm so that the lengths of the diagonals were as prescribed by three different variants shown in Table 1. The angle of the
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Fig. 4. Stimuli for the ‘‘against illusion’’ condition of Experiment 2. In the experiment the diagonals were printed in red.
diagonals in both figures was kept constant at exactly 43 tilted towards the grasping hand. 3.1.4. Design The design was exactly the same as in Experiment 1 except that the manual judgment condition replaced the verbal comparative length judgments. This made it possible to copy the design of the grasping condition for the judgment condition. 3.2. Results 3.2.1. Indicating length (judgments) We analysed both hands separately to make a direct comparison with the grasping condition possible. For the dominant hand an analysis of variance was carried out on the mean fingerthumb aperture over the four or eight (in the ‘‘against condition’’) trials of each condition with the factor task order (matching first versus grasping first) between participants and factors condition (‘‘against’’ vs. ‘‘neutral’’ vs. ‘‘with illusion’’) and figure (larger vs. smaller) as within participants factors. The analysis revealed a significant main effect for factor figure (F (1, 16) = 48.42, p < .001). Finger-thumb aperture was larger for the larger figure in all of the three conditions. Furthermore, there was a significant interaction between figure and condition (F (2, 32) = 5.47, p < .01). Fig. 5A shows that the interaction comes about because finger-thumb aperture depended on actual size difference (condition) and effects of the illusion (figure). In fact the influence of the illusion was so strong that even in the ‘‘against’’ condition it overpowered the actual length difference in the opposite direction: t (17) = 2.11, p = .05. The same analysis of variance for the non-dominant hand showed the same effects for figure (F (1, 16) = 18.49, p < .01) and the interaction between figure and condition (F (2, 32) = 5.67, p < .01) significant. As Fig. 5B shows the means look very similar to those from the dominant hand. However, the illusion was not strong enough make a significant difference in the ‘‘against’’ condition (t(17) = .73, p = .48), only in the ‘‘neutral’’ condition (t(17) = 3.35, p < .01). 3.2.2. Grasping Again, the grasping movements did not show the typical maximum grip aperture. For this reason, we analysed the pictures at the point where the hands had gone through about 70% of their trajectory. As in the first experiment, the dominant hand behaved quite differently from the non-dominant hand. Although there was only a very small difference in grip size at the point
B
distance between thumb and index finger
A
8,2
distance between thumb and index finger
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8,2
p = .004
281
p = .000
8 7,8 Figure
p = .001
7,6
large small
7,4 7,2 7 6,8
against
zero Condition
with
p = .002 8
p = .447
7,8 p = .004
Figure
7,6
large
7,4
small
7,2 7 6,8
against
zero Condition
with
Fig. 5. (A) Mean finger-thumb aperture as indicator of perceived length in Experiment: dominant hand. (B) Mean finger-thumb aperture as indicator of perceived length in Experiment: non-dominant hand
of measurement between dominant hand (5.62 cm) and non-dominant hand (5.50 cm), the grip size of the non-dominant hand showed again no interpretable relationship with the stimulus conditions. To illustrate, even when the actually longer diagonal was made to look even longer in the ‘‘with’’ illusion condition was the average grip size was smaller (5.67 cm) than for the shorter diagonal (5.80 cm), though non-significantly so: t (17) = 1.49, p > .05. We, therefore, only analysed the data from the dominant hand in detail. The same analysis of variance was carried out as on the manual judgment data described above. The only significant effect was the interaction between figure and condition (F (2, 30) = 11.51, p < .001). The relevant means in Fig. 6 show that the interaction comes about because grip size differs significantly in the direction of the true length of the diagonals. In the ‘‘with’’ illusion condition there is a highly significant difference (t (17) = 5.37, p < .001). There is practically no difference when diagonals are the same length (t (16) = .66, p > .50). And, most importantly, grip size differs with actual length of diagonals even when it goes against the illusion (t (17) = 3.31, p < .01).
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grip aperture
5,7
p = .000 p = .004
p = .520 Figure
5,65
large
5,6 5,55
small
5,5 5,45 5,4 5,35 5,3 against
zero Condition
with
Fig. 6. Mean grip size for dominant hand in Experiment 2.
3.2.3. Susceptibility to the illusion We also computed scores for the susceptibility to the illusion in the grasping and in the judgment condition. For grasping trials we used the same measure as in Experiment 1 and for judgment trials we computed the corresponding measure as for grasping. For the dominant hand the Pearson correlation coefficient for these two susceptibility measures was non-significantly positive: r = .10, p = .71. This provides yet again no evidence for the single representation model. Moreover, the mean susceptibility score in the grasping condition was .032 cm which is not significantly different from zero (t (16) = .81, p = .43).1 In contrast, the mean susceptibility score for the judgment condition was .55 cm which is highly significantly different from zero (t (17) = 7.21, p < .001). Comparison of the two scores makes clear that susceptibility to illusion was much greater for judgments than for grasping: t (16) = 6.04, p < .001. A similar result emerged for judgement by the non-dominant hand. The susceptibility score of 0.46 cm was highly significant (t (17) = 3.71, p < .002). 3.3. Discussion The grasping data closely replicate the data from Experiment 1. Again grip size scaling on the non-dominant hand was insensitive to the stimulus differences (even more so than in Experiment 1), while the dominant hand behaved much more disciplined and reflected stimulus differences minutely. There was no indication that the illusion affected the dominant handÕs grip scaling. In stark contrast, when finger-thumb scaling was used as conscious estimates of stimulus lengths, estimates always went significantly in the direction of the illusion even in the ‘‘against illusion’’ condition, where the actual lengths differed in the opposite direction. Knowing from Experiment 1 that despite the need for bimanual grasping the non-dominant hand showed such imprecise grip size scaling, we expected a similar finding for the finger-thumb 1
There is a change in the degrees of freedom because data of one participant are incomplete.
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scaling as indicators of perceived size. To our surprise, in this task the non-dominant hand produced results comparable to the dominant hand, showing comparable sensitivity to stimulus differences, though the influence of the illusion was not as strong. This indicates that, despite the apparent parallelism in task demands between grasping and conscious judgment by thumb-finger span, there still remained some attentional asymmetry between the grasping and judgment task.
4. General discussion The main objective in this study was to find illusory stimuli, which do not require visual ‘‘obstacles’’ in the path of the grasping hand, to avoid extraneous reasons for grip size scaling apart from the length of the to be grasped object. Such extraneous reasons can have potential effects that can be exploited by opposing interpretations. As Jacob and Jeannerod (2003) point out, they detract from the interpretation that grasping is less influenced by illusions than conscious judgment, because the observed difference could be due to such extraneous reasons rather than a difference in internal representation. Such visual obstacles may, however, also lead to the opposite effect of changing the grip scaling in the direction of the illusion. The data from Haffenden and Goodale (1998, 2000) provide concrete evidence for this danger in the Ebbinghaus/Titchener illusion. But also the Mu¨ller-Lyer illusion may be particularly prone to this, because the arrow wings at the end of the stick may be perceived by the motor system as belonging to the object to be grasped. Consequently, the grip size will be larger for these objects but for a reason different from the illusory effect created by these arrow wings. In sum, because such extraneous reasons were avoided in our stimuli our results provide improved support for the dual representation theory. There is little indication that grip size scaling of the dominant hand is subject to the illusion (and the non-dominant hand showed some signs of being influenced only in Experiment 1). In contrast, comparisons judgments in Experiment 1 and manual estimates on both hands in Experiment 2 as measures of conscious perception showed very strong effects of the illusion.
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